INDIAN HEAD, Md. — More than 40 expensive houses under construction in Charles County were burned early yesterday in a development that has drawn criticism from environmentalists because it is next to a nature preserve.

Arson is suspected in at least four of the 41 blazes, a state fire official said. The houses, 12 of which were destroyed, were priced at $400,000 to $500,000.

Ecoterrorism is one of the motives that would be investigated, said Joe Parris, a spokesman for the FBI, which joined the investigation last night.

“From what you can see, it would appear that way, but we’re not ready to start pointing the finger yet,” Mr. Parris said. “Right now, it’s a local investigation, and the FBI is not willing to speculate into individuals or groups that might be responsible.”

There was no immediate claim of responsibility.

No injuries were reported in the fires in the Hunters Brooke development off Route 225, next to the state’s Mattawoman Natural Environment Area.

Damage was estimated at at least $10 million. If the fires are deemed to be acts of ecoterrorism, it will be one of the most expensive known acts of ecoterrorism in the United States.

Deputy State Fire Marshal W. Faron Taylor refused to disclose what led investigators to suspect arson in four of the fires.

“At this point, our knowledge of the methodology is shared by us and the perpetrator, and we don’t want to share that with anyone else,” he said. “We’re not going to tip our hand.”

The blazes were reported before 5 a.m., drawing more than 100 firefighters from Charles, Prince George’s, Calvert and St. Mary’s counties to the 319-unit subdivision about 25 miles south of the District.

The houses, on lots of about a quarter-acre each, were spread across a 10-acre area, Marshal Taylor said.

“This was a very, very affluent neighborhood under construction,” he said. “I have never in 20 years seen the magnitude of destruction that I have seen today.”

The houses were in varying stages of development, and the subdivision was largely unoccupied. A few houses were completed, while wooden frames were just erected on others.

Only a few houses were occupied, but none of those was burned down. The occupants escaped the area unharmed.

Antoine Potts, 14, who lived in one of the few occupied houses, first detected the fires burning two houses away.

“I woke up to a smell, like a barbecue smell,” he said.

He woke up his parents and two siblings, ages 10 and 3, and they rushed out of their home.

“As we got up, we could see fire everywhere,” Antoine said.

Antoine’s father, Derrick, and mother, Terri Rookard, drove the children to safety, passing by dozens of burning houses.

Jacque Hightower was one of the people looking forward to moving into a new home in the subdivision.

“We were going to settlement in about two days, and we woke up this morning and looked at the news, and my wife just screamed, ‘Our house is on fire,’ so I just came down today to try to find out what’s going on,” Mr. Hightower told WRC-TV (Channel 4).

Developers had hired a security company to protect the houses, but residents said the security officers left the area at about 4 a.m.

In addition to the FBI, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives are assisting local authorities in their investigation. An estimated 30 investigators were on the scene throughout the day.

The number of homes and the distance between the homes prompted speculation that the fires might have been intentionally set as an act of ecoterrorism.

Environmental groups such as the Sierra Club and a citizens group opposed the Hunters Brooke development and another subdivision because of their proximity to the Araby Bog, a 50-acre magnolia bog.

Magnolia bogs are wetland areas that provide a unique environment for plants and animals, and the Araby Bog is one of the largest and most pristine bogs in the mid-Atlantic region, conservationists said.

The Sierra Club said yesterday it condemned “all acts of violence in the name of the environment.”

“The perpetrators should be prosecuted to the full extent of the law,” said Bruce Hamilton, the group’s conservation director. “That type of criminal behavior does nothing to further the cause of promoting safe and livable communities.”

In recent years, the Earth Liberation Front (ELF), a loosely structured group that opposes commercialism and industry in the name of preserving the environment, has taken responsibility for many similar incidents nationwide.

ELF did not respond to an e-mail sent by The Washington Times to its spokesman yesterday.

In the past several years, ELF has burned down apartment complexes in San Diego and a ski resort in Colorado, vandalized sport utility vehicles at dealerships in Virginia, and destroyed construction sites and equipment in Pennsylvania. Total damage caused by the group is estimated at more than $100 million.

In 2002, a top FBI domestic terrorism official called ELF and the Animal Liberation Front, an animal rights group, the largest and most active U.S.-based terrorist organizations. No one has been injured in any of the incidents.

The Sierra Club called the Charles County development “quintessential sprawl” in its fall 2000 sprawl report, noting that it is far from existing infrastructure and “threatens a fragile wetland and important historical sites near the Chesapeake Bay.”

In 2003, a group of residents and environmental protection groups brought a suit against the Army Corps of Engineers, which authorized a permit for a sewer system and a road crossing as part of the Hunters Brooke subdivision, built by Lennar Homes of La Plata.

Arguing that the subdivision would cause severe environmental damage to the Araby Bog, the plaintiffs said the Corps’ permission violated the Clean Water Act’s restriction that such a permit be granted only when minimal harm would be done to the environment.

Represented by Georgetown University Law Center’s Institute for Public Representation (IPR), the plaintiffs received only a partial victory this summer when a judge remanded the case to the Corps, allowing the agency to provide a more detailed explanation for the merits of the permit.

Kristi M. Smith, a staff lawyer with IPR, said construction was not affected by the judge’s order and that the Corps was not likely to issue its explanation until early next year.

Patricia Stamper, a plaintiff in the lawsuit and co-chairwoman of the citizens’ group known as the Save Araby Mattawoman and Mason Springs, said although her group has fought the development’s location near the bog, it was not involved in the arson.

“The fires came as a total surprise,” she said. “I don’t know who would have done such a thing.”

 Sarah Hoffman contributed to this article, which is based in part on wire service reports.